Hydrocarbons (oil, natural gas, etc.) are obtained from a subterranean geological formation, or reservoir, by drilling a well that penetrates the hydrocarbon-bearing formation. This provides a partial flowpath for the hydrocarbon to reach the surface. In order for the hydrocarbon to be produced, that is travel from the formation to the wellbore and ultimately to the surface, a sufficiently unimpeded flowpath should be formed from the formation to the wellbore.
Hydraulic fracturing may improve well productivity by extending reservoir contact between the borehole and the reservoir. This operation may be performed by hydraulically injecting a fracturing fluid into a wellbore penetrating the formation and forcing the fracturing fluid against the formation strata by pressure. The formation strata or rock is forced to crack and fracture, thereby increasing flow paths between the reservoir and the borehole. Proppant may be placed in the fracture to prevent the fracture from closing and thus, provide improved flow of the recoverable hydrocarbons.
Hydraulic fracturing for well stimulation includes pumping the fracturing fluid at a bottomhole pressure sufficient to overcome the formation in-situ stresses so that the rock can be cracked. An effective bottomhole pressure may be the sum of the surface pressure provided by the pumping equipment and the hydrostatic pressure, minus the pressure losses due to friction forces while the fluid passes through the surface and subterranean equipments such as pipes. The bottomhole pressure may be governed by the mechanical properties of the formation, and may be higher as the borehole extends deeper.
Once a fracture is initiated, enough bottomhole pressure may be maintained to propagate the fracture further away from the wellbore and generate the necessary fracture width for it to be filled with the propping material that will keep the fracture open once the pumping has stopped. The initial breakdown pressure may be higher than the minimum pressure needed to re-open the same fracture due to geomechanical effects in the near well bore region as the far-field stress state interacts with the void space created by the drilling activity.